The present invention relates to printing apparatus and methods, and more particularly to imaging of lithographic printing-plate constructions on- or off-press using controlled laser output.
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
To circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
For example, U.S. Pat. No. 5,493,971 discloses wet-plate constructions that extend the benefits of ablative laser imaging technology to traditional metal-based plates. Such plates remain the standard for most of the long-run printing industry due to their durability and ease of manufacture. As shown in FIG. 1, a lithographic printing construction 100 in accordance with the ""971 patent includes a grained-metal substrate 102, a protective layer 104 that can also serve as an adhesion-promoting primer, and an ablatable oleophilic surface layer 106. In operation, imagewise pulses from an imaging laser (typically emitting in the near-infrared, or xe2x80x9cIRxe2x80x9d spectral region) interact with the surface layer 106, causing ablation thereof and, probably, inflicting some damage to the underlying protective layer 104 as well. The imaged plate 100 may then be subjected to a solvent that eliminates the exposed protective layer 104, but which does no damage either to the surface layer 106 or to the unexposed protective layer 104 thereunder. By using the laser to directly reveal only the protective layer and not the hydrophilic metal layer, the surface structure of the latter is preserved; the action of the solvent does not damage this structure.
This construction relies on removal of the energy-absorbing layer to create an image feature. Exposure to laser radiation may, for example, cause ablationxe2x80x94i.e., catastrophic overheatingxe2x80x94of the ablated layer in order to facilitate its removal. Accordingly, the laser pulse must transfer substantial energy to the absorbing layer. This means that low-power lasers must be capable of very rapid response times, and imaging speeds (i.e., the laser pulse rate) must not be so fast as to preclude the requisite energy delivery by each imaging pulse.
In order to reduce or even obviate the need for substantial ablation as an imaging mechanism, U.S. application Ser. No. 09/564,898, now U.S. Pat. No. 6,378,432, the entire disclosure of which is hereby incorporated by reference, discloses a construction combining the benefits of simple construction, the ability to utilize traditional metal base supports, and amenability to imaging with low-power lasers that need not impart ablation-inducing energy levels. As shown in FIGS. 2A-2C and 3A-3B, in one embodiment, a printing member includes a hydrophilic metal substrate 302, a topmost layer 306 that does not significantly absorb imaging radiation, and an intermediate layer 304 that does absorb imaging radiation. The radiation-absorbing layer 304 comprises a radiation-absorptive material (which may be graded through the thickness of layer 304 if desired). In one version as shown in FIGS. 2A-2C, in response to an imaging pulse the absorbing layer 304 debonds from the surface of the adjacent metal substrate; in another version as shown in FIGS. 3A-3B, an interior split is formed within the absorbing layer, facilitating removal of the portion of that layer above the split. In neither case does the absorbing layer undergo substantial ablation. Remnants of the absorbing layer and the overlying layer (or layers) are readily removed by post-imaging cleaning to produce a finished printing plate.
The cost of manufacturing a printing plate is generally a function of the number of plate layers. Because each layer is individually applied in a separate process step, elimination of a layer can materially reduce overall production costs. In accordance with the present invention, the functions performed by layers 304 and 306 are combined into a single layer.
In particular, the present invention provides a printing member having a single radiation-absorptive multiphase layer over a substrate layer that may be imaged with or without ablation. The multiphase layer may be in contact with the substrate layer along an interface. The multiphase layer comprises a polymer-rich phase and an inorganic-rich phase dispersed within the polymer-rich phase. To provide a lithographic image, the printing member is subjected to imaging radiation in an imagewise pattern. The radiation removes or facilitates removal of at least a portion of the multiphase layer but does not affect the substrate. Following imaging, a cleaning step may be used to remove remnants of the portion of the multiphase layer, thereby creating an imagewise lithographic pattern on the printing member. The printing member may now be used for printing.
In preferred embodiments, a printing member in accordance with the invention comprises a multiphase layer and a substrate. In one embodiment, the substrate is a metal substrate. Suitable metal substrates include, but are not limited to, aluminum, copper, steel, and chromium. In a preferred embodiment, the metal substrate is grained, anodized, and/or silicated. For example, the substrate may be aluminum. In another embodiment, the substrate is a polymer substrate. Suitable polymer substrates include, but are not limited to, polyesters, polycarbonates, and polystyrene. In a preferred embodiment, the substrate is a polyester film, and preferably a polyethylene terephthalate film. In still another embodiment, the substrate is a paper substrate.
The multiphase layer may comprise a polymer-rich phase and an inorganic-rich phase. Suitable materials for the polymer-rich phase include, but are not limited to, polyvinyl alcohols, copolymers of polyvinyl alcohol, polyvinyl pyrrolidone and its copolymers, and polyvinylether and copolymers thereof. In a preferred embodiment, the polymer is a polyvinyl alcohol. The inorganic-rich phase contains one or more inorganic oxides, typically formed as a reaction product of an initially soluble complex. Such inorganic oxides may include, for example, zirconium oxide (typically ZrO2), aluminum oxide (typically Al2O3), silicon dioxide and titanium oxide (typically TiO2), as well as combinations and complexes thereof. It should also be noted that these oxides may exist in hydrated form. In a preferred embodiment, the inorganic-rich phase comprises xe2x80x9cnodulesxe2x80x9d rich in zirconium oxide. Preferably, the nodules are dispersed within the polymer-rich phase. In one embodiment, the inorganic-rich phase further comprises an inorganic-rich interfacial layer at the interface of the multiphase layer with the metal substrate. In a preferred embodiment, the interfacial layer comprises zirconium oxide, and may have a thickness of 5 nm or less.
In preferred embodiments, the multiphase layer comprises a material that absorbs imaging radiation. In one embodiment, the absorptive material renders the multiphase layer subject to ablative absorption of imaging radiation. Thus, the imaging mechanism is ablative in nature, whereby at least a portion of the multiphase layer is destroyed by the laser pulse. For example, laser radiation may remove or facilitate removal of a portion of the multiphase layer above the inorganic-rich interfacial layer. Alternatively, laser radiation may remove or facilitate removal of the entire multiphase layer. In another embodiment, the imaging mechanism is non-ablative in nature. For example, the laser pulse may merely debond a portion of the multiphase layer from the inorganic-rich interfacial layer. Alternatively, the laser radiation may debond the entire multiphase layer from the substrate without substantially ablating the layer. In these cases, the debonded material may then be removed by post-imaging cleaning (see, e.g., U.S. Pat. Nos. 5,540,150; 5,870,954; 5,755,158; and 5,148,746).
The polymer-rich phase of the multiphase layer has a different affinity at least from the substrate for a printing liquid such as an ink or an ink-rejecting fluid. In one embodiment, the substrate is a hydrophilic metal substrate, while the polymer-rich phase is oleophilic. In this configuration, the inherently ink-receptive areas receive laser output and are ultimately removed, revealing the hydrophilic surface that will reject ink during printing. In other words, the xe2x80x9cimage areaxe2x80x9d is selectively removed to reveal the xe2x80x9cbackground.xe2x80x9d Such printing members are also referred to as xe2x80x9cpositive-workingxe2x80x9d or xe2x80x9cindirect-write.xe2x80x9d In one version of this embodiment, a portion of the multiphase layer is removed, leaving the exposed surface of the inorganic-rich interfacial layer to serve as the hydrophilic surface. Alternatively, the interfacial layer may be removed either during cleaning or use of the member in printing, exposing the underlying hydrophilic metal substrate.
In another embodiment, the substrate is oleophilic, while the polymer-rich phase is hydrophilic. This configuration results in a xe2x80x9cnegative-workingxe2x80x9d or xe2x80x9cdirect-writexe2x80x9d printing member. In this case, the entire multiphase layer is removed, exposing the oleophilic polymer substrate. The unexposed hydrophilic surface remains receptive to ink-rejecting fluids.
It should be understood that, as used herein, the term xe2x80x9cplatexe2x80x9d or xe2x80x9cmemberxe2x80x9d refers to any type of printing member or surface capable of recording an image defined by regions exhibiting differential affinities for ink and/or an ink abhesive fluid. Suitable configurations include the traditional planar or curved lithographic plates that are mounted on the plate cylinder of a printing press, but can also include seamless cylinders (e.g., the roll surface of a plate cylinder), an endless belt, or other arrangement.
Furthermore, the term xe2x80x9chydrophilicxe2x80x9d is used in the printing sense to connote a surface affinity for a fluid which prevents ink from adhering thereto. Such fluids include water for conventional ink systems, aqueous and non-aqueous dampening liquids, and the non-ink phase of single-fluid ink systems. Thus, a hydrophilic surface in accordance herewith exhibits preferential affinity for any of these materials relative to oil-based materials.